Insulated gate semiconductor device

ABSTRACT

A trench IGBT is disclosed which meets the specifications for turn-on losses and radiation noise. It includes a p-type base layer divided into different p-type base regions by trenches. N-type source regions are formed in only some of the p-type base regions. There is a gate runner in the active region of the trench IGBT. Contact holes formed in the vicinities of the terminal ends of the trenches and on both sides of the gate runner electrically connect some of the p-type base regions that do not include source regions to an emitter electrode. The number N 1  of p-type base regions that are connected electrically to the emitter electrode and the number N 2  of p-type base regions that are insulated from the emitter electrode are related with each other by the expression 25≦{N 1/ (N 1+ N 2 )}×100≦75.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/993,146, filed Nov. 19, 2004, which claims priority from applicationSerial No. JP 2003-390433 filed on Nov. 20, 2003 and JP 2004-224777,filed on Jul. 30, 2004, and the contents of these documents areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to insulated gate semiconductordevices and, more particularly, to trench-type insulated gate bipolartransistors (hereinafter referred to as “trench IGBTs”), which includean insulated gate structure in the trenches formed in a semiconductorsubstrate.

B. Description of the Related Art

Recently, trench IGBTs have been attracting much attention in the fieldof power devices used for electric power converters. Since the trenchIGBT facilitates increased trench density, the voltage drop V_(CE(sat))in the ON-state of the trench IGBT is small and the steady state lossesare reduced. However, since the capacitance between the gate electrodeand the emitter electrode, and the capacitance between the gateelectrode and the collector electrode (hereinafter referred to as the“gate-collector capacitance”), are large, large switching losses arecaused by the turning-on and turning-off of the trench IGBT.

It has been reported that the tradeoff relation between the saturationvoltage and the turn-off losses in the trench IGBT is reduced bydisposing p-type well regions not in electrical contact with the emitterelectrode to increase the accumulated carrier density on the emitterelectrode side (cf. JP P2000-228519A at page 4, left-hand column, lastline). Trench IGBTs including p-type well regions not in electricalcontact with the emitter electrode also have been disclosed in JPP2001-308327A (FIGS. 1 and 7), JP PHei.9(1997)-331063A (FIG. 42), JPP2002-100770A (FIG. 22), and JP P2002-16252A (FIG. 1).

FIG. 18 is a top plan view schematically showing a trench IGBT having astructure as described above. FIG. 19 is a cross-sectional view of thetrench IGBT along the line segment A-A of FIG. 18. In FIG. 18, p-typebase regions 9 and 10, n-type source regions 3, gate electrodes 5, andgate runners 13 and 14 are shown. Gate insulator films 4, interlayerinsulator films 6, and emitter electrode 7 are not shown in FIG. 18. InFIG. 19, the cross section along the line segment A-A, that is, thestructure across n-type source regions 3 and gate electrodes 5, is showntogether with the constituent elements not shown in FIG. 18.

As shown in FIGS. 18 and 19, n-type drift layer 2 is on p-type collectorlayer 1 and p-type base layer 20 is on n-type drift layer 2. P-type baselayer 20 is divided into p-type base regions 9 and 10 by trenches 21.N-type source region 3 is on the side of trench 21 in narrow p-type baseregion 9. An n-type source region 3 is not disposed in wide p-type baseregion 10.

Emitter electrode 7 is in contact with n-type source regions 3 andp-type base region 9. Emitter electrode 7 is insulated by interlayerinsulator film 6 from p-type type base region 10 including no n-typesource region 3. Trench 21 is filled with gate electrode 5 with gateinsulator film 4 interposed between them. As shown in FIG. 18, gateelectrodes 5 are connected electrically to gate runners 13 extendingacross the terminal ends of trenches 21. Gate runners 13 are connectedto a gate pad (not shown).

In the above-described structure including only gate runners 13 acrossboth ends of trenches 21, the gate electrode resistance between gaterunners 13 and the center of the active region, in which the maincurrent of the semiconductor device is made to flow, increases as thechip size increases. To obviate this problem, gate runners 14 aredisposed in the active region with a spacing of between 2 and 4 mm.Although not shown in the figures, a structure for sustaining thebreakdown voltage including guard rings and such means is disposedaround the active region.

FIG. 20 is a top plan view schematically showing the other conventionaltrench IGBT. FIG. 21 is a cross sectional view of the other trench IGBTalong the line segment B-B of FIG. 20. In the trench IGBT shown in FIGS.20 and 21, p-type base layer 20 is divided into p-type base regions 9and 12 by trenches 21. Emitter electrode 7 is in contact also withp-type base region 12, not including any p-type source region therein,via contact holes 11 formed through interlayer insulator film 6 (cf JPP2001-308327A). Contact hole 11 is 2 μm×2 μm in cross sectional area,and is disposed near the terminal end of trench 21.

In FIG. 20, p-type base regions 9 and 12, n-type source regions 3, gateelectrodes 5, gate runners 13 and 14, and contact holes 11 projected tothe surfaces of p-type base regions 12 are shown. Gate insulator films4, interlayer insulator films 6, and emitter electrode 7 are not shownin FIG. 20. In FIG. 21, the cross section along the line segment B-B ofFIG. 20, that is the structure across n-type source regions 3, gateelectrodes 5 and contact holes 11, is shown together with theconstituent elements not shown in FIG. 20.

By optimizing the surface structure including trenches 21, that is byoptimizing the surface structure including gate electrodes 5, it ispossible for the trench IGBTs described above to realize low steadystate losses and low switching losses (high speed switching)simultaneously. The trench IGBT, having the structure described in FIG.21, facilitates preventing the breakdown voltage from decreasing ascompared with the trench IGBT having the structure described in FIG. 19.

When the chip size for any of the trench IGBTs described above is solarge that it is necessary to dispose gate runner 14 in the activeregion, the result is a large gate-collector capacitance across theboundaries between p-type base regions 10 or 12 not including any sourceregion 3 and gate insulator films 4 in the central part of the activeregion. Since the voltage drop speed and the current increase speedbecome slow in the turning-on of the trench IGBTs due to the largegate-collector capacitance, large turn-on losses are caused. Forpreventing the turn-on losses from increasing, it is necessary toimprove the gate-voltage change-over capabilities of the switchingdevices for gate driving or the ICs for gate driving. Therefore, it isimpossible to use the conventional devices for gate driving.

Recently, it has been required to reduce the radiation noise caused byswitching for power devices. To reduce the radiation noise, it isnecessary to reduce the voltage drop speed (dV/dt) and the currentincrease speed (di/dt). Therefore, it is hard to reduce the radiationnoises and the switching losses simultaneously. Since a tradeoff existsbetween the turn-on losses and the radiation noise as described above,it is impossible for the trench IGBTs having any of the conventionalstructures to obtain an optimum structure that meets the specificationsfor turn-on losses and radiation noise.

It has been reported that the device characteristics exhibited by theIGBT turning on at a low current of about one-tenth the rated currentgreatly affects radiation noise (S. Momota, et al. “Analysis on the LowCurrent Turn-On Behavior of IGBT Modules,” Proc. ISPSD2000, 359-362(2000)). Tremendous efforts are necessary to suppress the radiationnoise caused, especially that in the frequency range of 30 MHz or higherbelow the reference level. It has been reported that radiation noise iscaused in the frequency range of 30 MHz or higher by a high (dV/dt)containing high frequency components. To suppress the (dV/dt) in theswitching of an inverter below the reference value, the gradient of themain current (dI_(c)/dt) is suppressed at a low value by adjusting thegate resistance and such parameters.

However, high gate resistance increases the turn-on losses of the IGBT.FIG. 11 is a wave chart describing the simulation results for comparingthe turn-on characteristics of the IGBT shown in FIG. 8 and theconventional IGBTs with the gate resistance values thereof changed. Asshown in FIG. 11, the gradient (di/dt) of the current during the turn-onof the IGBT (hereinafter referred to as the “turn-oncurrent-change-speed”) is reduced by increasing the gate resistance.Although the gate resistance increase is preferable to reduce theradiation noise, switching losses increase, since the gate resistanceincrease causes long tails in the voltage waveforms. Therefore, it ispreferable for the trench IGBT to realize a low (di/dt) whilesuppressing the gate resistance as low as possible.

In view of the foregoing, it would be desirable to obviate the problemsdescribed above. It would be also desirable to provide an insulated gatesemiconductor device that facilitates meeting the specifications on theturn-on losses and the switching noises simultaneously. The presentinvention is directed to overcoming or at least reducing the effects ofone or more of the problems set forth above.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided aninsulated gate semiconductor device including a first semiconductorlayer of a first conductivity type; a second semiconductor layer of asecond conductivity type on the first semiconductor layer; a thirdsemiconductor layer of the first conductivity type on the secondsemiconductor layer; and trenches formed through the third semiconductorlayer down to the second semiconductor layer. The third semiconductorlayer is divided into multiple semiconductor regions by the trenches.Fourth semiconductor layers of the second conductivity type selectivelyare formed at least in the surface portions of some of the semiconductorregions. A control electrode is formed in each of the trenches with aninsulator film interposed therebetween. There is a runner on the thirdsemiconductor layer in the active region, therein the semiconductordevice makes a current flow. There is an insulator film interposedbetween the runner and the third semiconductor layer, with the runnerbeing connected electrically to the control electrodes. There is a firstmain electrode on the third and fourth semiconductor layers with aninterlayer insulator film interposed therebetween, and a second mainelectrode is connected electrically to the first semiconductor layer.The first main electrode is in contact with the third semiconductorlayer and the fourth semiconductor layers in the semiconductor regions,including the fourth semiconductor layers formed therein, through theinterlayer insulator film. The first main electrode also is inelectrical contact with the third semiconductor layer in some of thesemiconductor regions, not including any fourth semiconductor layer, viacontact holes formed in the vicinities of the terminal ends of thetrenches and in the vicinities of the runner through the interlayerinsulator film.

According to the first aspect of the invention, the first main electrodecontacts, in the vicinities of the runner disposed in the active region,with some of the semiconductor regions formed by dividing the thirdsemiconductor layer, but not including any fourth semiconductor layertherein. This configuration facilitates reducing the gate-collectorcapacitance caused across the boundaries between the third semiconductorlayer in the semiconductor regions not including any fourthsemiconductor layer and the insulator films in the trenches andincreasing the voltage drop speed and the current increase speed duringthe turn-on of the device. Therefore, the turn-on losses are reduced.

Advantageously, the other semiconductor regions, not including anyfourth semiconductor layer, excluding the some of the semiconductorregions, not including any fourth semiconductor layer but in contactwith the first main electrode, are insulated from the first mainelectrode by the interlayer insulator film.

The semiconductor regions, formed by dividing the third semiconductorlayer by the trenches and contacting only with the first main electrodeelectrically, and the semiconductor regions, formed by dividing thethird semiconductor layer by the trenches and insulated from the firstmain electrode, are arranged appropriately so that the turn-on lossesand the radiation noise may be reduced.

Advantageously, the number N1 of the some of the semiconductor regions,not including any fourth semiconductor layer but in contact with thefirst main electrode, and the number N2 of the other semiconductorregions, not including any fourth semiconductor layer but insulated fromthe first main electrode, are related with each other by the relationalexpression 0.25≦N1/(N1+N2)≦0.75. By setting the numbers N1 and N2 suchthat these conditions are met, the turn-on losses and the radiationnoise are reduced.

According to a second aspect of the invention, there is provided aninsulated gate semiconductor device including a first semiconductorlayer of a first conductivity type, a second semiconductor layer of asecond conductivity type on the first semiconductor layer, and a thirdsemiconductor layer of the first conductivity type on the secondsemiconductor layer. Trenches are formed through the third semiconductorlayer down to the second semiconductor layer, the third semiconductorlayer being divided by the trenches at least into first semiconductorregions, second semiconductor regions, and third semiconductor regions.Fourth semiconductor layers of the second conductivity type selectivelyare formed at least in the surface portions of the first semiconductorregions. A control electrode is formed in each of the trenches with aninsulator film interposed therebetween. A runner is on the thirdsemiconductor layer in the active region, therein the semiconductordevice makes a current flow, and there is an insulator film interposedbetween the runner and the third semiconductor layer, the runner beingconnected electrically to the control electrodes. A first main electrodeis on the third and fourth semiconductor layers with an interlayerinsulator film interposed between them, and a second main electrode isconnected electrically to the first semiconductor layer. The first mainelectrode is in contact with the third semiconductor layer and thefourth semiconductor layers through the interlayer insulator film in thefirst semiconductor regions. The first main electrode is in contact withthe third semiconductor layer only via contact holes formed through theinterlayer insulator film in the second semiconductor regions, and thecontact holes are formed in the vicinities of the terminal ends of thetrenches and in the vicinities of the runner. The first main electrodeis insulated from the third semiconductor layer and the fourthsemiconductor layers by the interlayer insulator film in the thirdsemiconductor regions.

In the configuration described above, the first main electrode contactswith the third semiconductor layer only via the contact holes in thevicinities of the runner disposed in the active region and in thevicinities of the terminal ends of the trenches. This configurationfacilitates reducing the gate-collector capacitance caused across theboundaries between the third semiconductor layer and the insulator filmsin the trenches and increasing the voltage drop speed and the currentincrease speed during the turn-on of the device. Therefore, the turn-onlosses are reduced.

Advantageously, the number N1 of the second semiconductor regions andthe number N2 of the third semiconductor regions are related with eachother by the relational expression 0.25≦(N1/(N1+N2)≦0.75. By setting thenumbers N1 and N2 such that these conditions are satisfied, the turn-onlosses and the radiation noise are reduced.

According to a third aspect of the invention, there is provided aninsulated gate semiconductor device including a first semiconductorlayer of a first conductivity type, a second semiconductor layer of asecond conductivity type on the first semiconductor layer, and a thirdsemiconductor layer of the first conductivity type on the secondsemiconductor layer. Trenches are formed through the third semiconductorlayer down to the second semiconductor layer, dividing the thirdsemiconductor layer into at least into relatively narrow semiconductorregions and relatively wide semiconductor regions. Fourth semiconductorlayers of the second conductivity type selectively are formed at leastin the surface portions of the relatively narrow semiconductor regions.A control electrode is formed in each of the trenches with an insulatorfilm interposed therebetween. A first main electrode is on the third andfourth semiconductor layers with an interlayer insulator film interposedtherebetween, and a second main electrode connects electrically to thefirst semiconductor layer. The first main electrode is in contact withthe third semiconductor layer and the fourth semiconductor layersthrough the interlayer insulator film in the relatively narrowsemiconductor regions. The third semiconductor layer is connectedelectrically to the first main electrode via resistance of 50 mΩ orhigher in the relatively wide semiconductor regions.

Advantageously, the resistance is the sheet resistance of an impuritylayer in the relatively wide semiconductor region, and the impuritylayer is connected electrically to the first main electrode via contactholes formed locally through the interlayer insulator film. Also, it ispreferable to shape the trenches as respective stripes extending inparallel to each other and the contact holes are aligned along thestripe-shaped trenches with spacing between 200 μm and 2 mm. In apreferred embodiment, the resistance is made of doped polysilicon.

Since the above described advantageous configurations reduce di/dt bylow gate resistance, the turn-on losses and the radiation noises arereduced. The insulated gate semiconductor device according to theinvention facilitates reducing the turn-on losses and the radiationnoises. Therefore, an insulated gate semiconductor device having astructure that meets the specifications for turn-on losses and radiationnoise is obtained according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIG. 1 is a cross sectional view schematically showing a trench IGBTaccording to a first embodiment of the invention.

FIG. 2 is another cross sectional view of the trench IGBT according tothe first embodiment of the invention.

FIG. 3 is a top plan view schematically showing the trench IGBTaccording to the first embodiment of the invention.

FIG. 4 is a pair of curves relating the turn-on losses and the voltagechange speed during the turn-on of the device with the short circuitrate.

FIG. 5 is a set of curves relating the voltage change speed during theturn-on of the device with the gate resistance with the short circuitrate as a parameter.

FIG. 6 is a top plan view schematically showing a trench IGBT accordingto a second embodiment of the invention.

FIG. 7 is a cross sectional view of the trench IGBT according to thesecond embodiment of the invention.

FIG. 8 is a cross sectional view for describing the principle of atrench IGBT according to a third embodiment of the invention.

FIG. 9 is a graph describing the simulated relation between the peakcurrent reduction rate and the resistance value in the trench IGBThaving the structure shown in FIG. 8.

FIG. 10 is a graph describing the simulated relation between theon-voltage and the resistance value in the trench IGBT having thestructure shown in FIG. 8.

FIG. 11 is a wave chart showing the results of simulations for comparingthe turn-on characteristics of the IGBT shown in FIG. 8 and theconventional IGBTs with the gate resistance values thereof changed.

FIG. 12 is a perspective view schematically showing the structure of thetrench IGBT according to the third embodiment of the invention.

FIG. 13 is a graph describing the experimental results relating thecontact hole spacing and the peak current reduction rate in the trenchIGBT having the structure shown in FIG. 12.

FIG. 14 is a graph describing the experimental results relating thecontact hole spacing and the on-voltage in the trench IGBT having thestructure shown in FIG. 12.

FIG. 15 is a top plan view of a trench IGBT according to a fourthembodiment of the invention.

FIG. 16 is a cross sectional view of the trench IGBT according to thefourth embodiment.

FIG. 17 is another cross sectional view of the trench IGBT according tothe fourth embodiment.

FIG. 18 is a top plan view schematically showing a conventional trenchIGBT.

FIG. 19 is a cross sectional view of the conventional trench IGBT alongthe line segment A-A of FIG. 18.

FIG. 20 is a top plan view schematically showing the other conventionaltrench IGBT.

FIG. 21 is a cross sectional view of the other trench IGBT along theline segment B-B of FIG. 20.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Now the invention will be described in detail hereinafter with referenceto the accompanied drawing figures which illustrate the preferredembodiments of the invention. Throughout these figures, the samereference numerals are used to designate the same constituent elementsand their duplicated descriptions are omitted for the sake ofsimplicity. Although the first conductivity type is a p-type and thesecond conductivity type is an n-type in the following descriptions, thepresent invention is applicable to trench IGBTs in which the firstconductivity type is an n-type and the second conductivity type is ap-type.

First Embodiment

FIG. 3 is a top plan view schematically showing a trench IGBT accordingto a first embodiment of the invention. In FIG. 3, p-type base regions9, 10, and 12, n-type source regions 3, gate electrodes 5, gate runners13 and 14, and contact holes 11 projected to the surfaces of p-type baseregions 12 are shown. Gate insulator films 4, interlayer insulator films6, and emitter electrode 7 are not shown in FIG. 3.

FIG. 1 is a cross-sectional view along the line segment C-C of FIG. 3across n-type source regions 3, gate electrodes 5, and contact holes 11in the vicinity of gate runner 14 in the active region. FIG. 2 is across sectional view along the line segment D-D of FIG. 3 across gaterunner 14 in the active region and contact holes 11 on both sides ofgate runner 14. In FIGS. 1 and 2, the constituent elements not shown inFIG. 3 are shown.

As shown in FIGS. 1 through 3, n-type drift layer 2 (a secondsemiconductor layer) is on p-type collector layer 1 (a firstsemiconductor layer). P-type base layer 20 is a third semiconductorlayer and is on n-type drift layer 2. P-type base layer 20 is divided bytrenches 21 into first, second, and third p-type base regions 9, 12, and10.

First p-type base regions 9 include n-type source regions 3 and 3, whichare fourth semiconductor layers. In other words, n-type source region 3is in the surface portion of p-type base region 9 on the side of trench21. N-type source region 3 is formed neither in second p-type baseregion 12 nor in third p-type base region 10.

Emitter electrode 7, that is a first main electrode, is in contactcommonly with first p-type base regions 9 and n-type source regions 3 infirst p-type base regions 9. Emitter electrode 7 only is in contact withsecond p-type base regions 12 via contact holes 11 formed throughinterlayer insulator films 6. Third p-type base regions 10 are insulatedfrom emitter electrode 7 by interlayer insulator films 6. Trench 21 isfilled with gate electrode 5, working as a control electrode, withinterlayer insulator film 6 and gate insulator film 4 interposed betweenthem. Collector electrode 8, that is a second main electrode, is incontact with the back surface of p-type collector layer 1.

As shown in FIG. 3, gate electrodes 5 are connected electrically to gaterunners 13 and 13 extending across the end portions of trenches 21. Gaterunner 13 is made of the same material as the material of gate electrode5, such as polysilicon, the resistance of which is lowered by impuritydoping. Gate electrodes 5 are connected electrically to gate runner 14extending across trenches 21 in the active region of the device. Gaterunners 13 and 14 are connected electrically to a gate pad (not shown).

As shown in FIG. 2, gate runner 14 arranged in the active region isinsulated from second p-type base regions 12 by insulator films 4.Although not illustrated, gate runner 14 arranged in the active regionis insulated from first and third p-type base regions 9 and 10 byinsulator films 4. Although not illustrated, gate runner 14 is connectedelectrically to gate electrodes 5 at the respective cross pointsthereof.

As shown in FIGS. 2 and 3, contact holes 11 for connecting emitterelectrode 7 and second p-type base regions 12 electrically to each otherare arranged on both sides of gate runner 14 in the active region and inthe vicinities of the terminal ends of trenches 21. In the structureshown in FIG. 2, gate runner wiring 15 is disposed, for example, incontact with gate runner 14. Gate runner wiring 15 is disposed when theresistance of gate electrodes 5 is not low enough. Gate runner wiring 15and emitter electrode 7 are formed by pattering the same wiring layer.Although not illustrated, a structure for sustaining the breakdownvoltage including guard rings and such means is disposed around theactive region. The active region is the portion that functions as atransistor.

The phrase “terminal ends of the trenches” includes various shapes inaddition to those exemplified in the drawings, i.e., stripes having noconnection with one another and all stripes having a connection with oneanother. More particularly, the gate electrode 5 under gate runner 13 isnot necessarily located in the trench. It is necessary only that thegate runner 13 and the gate electrode 5 be electrically connected witheach other. Thus, the phrase “terminal ends of the trenches” moreparticularly refers to the terminal ends of the long sides of the activeregion.

Now the preferable ratio of the number of second p-type base regions 12connected electrically to emitter electrode 7 and the number of thirdp-type base regions 10 insulated from emitter electrode 7 will bedescribed below. The number N1 of second p-type base regions 12 and thenumber N2 of third p-type base regions 10 are related by the expression25≦{N1/(N1+N2)}×100≦75

In other words, the preferable ratio of second p-type base regions 12 isfrom 25% to 75% of the total number (N1+N2) of second and third p-typebase regions 12 and 10. Hereinafter, the ratio {N1/(N1+N2)} is referredto as the “short circuit rate.” It is preferable to set the shortcircuit rate between 25% and 75%.

When one second p-type base region 12 is arranged with respect to threethird p-type base regions 10, the short circuit rate is 25%. When threesecond p-type base regions 12 are arranged with respect to one thirdp-type base region 10, the short circuit rate is 75%. In FIGS. 1 and 3,the short circuit rate is 50%.

Now the reason why it is preferable to set the short circuit ratebetween 25% and 75% will be described below with reference to FIGS. 4and 5. FIG. 4 is a pair of curves relating the turn-on losses (Eon) andthe voltage change speed (dV/dt) during the turn-on of the device(hereinafter referred to as the “turn-on voltage-change-speed”) with theshort circuit rate. Referring now to FIG. 4, the voltage change speedincreases with increasing short circuit rate, and the turn-on lossesbecome lower with increasing short circuit rate.

FIG. 5 is a set of curves relating the turn-on voltage-change-speed(dV/dt) with the gate resistance with the short circuit rate as aparameter. It has been reported that the magnitude of the radiationnoise is affected by the turn-on voltage-change-speed and the turn-oncurrent-change-speed. Since the voltage change speed and the currentchange speed become high and low in synchrony with each other, thevoltage change speed will be used below as the characteristicsrepresenting the magnitude of the radiation noises.

Referring now to FIG. 4, when the short circuit rate is between 0 and25%, the turn-on losses increase sharply with decreasing short circuitrate and the lowering of the turn-on voltage-change-speed that affectsthe radiation noise greatly is small. To suppress the turn-on losses toa low level, it is necessary to improve the gate-voltage change-overcapabilities of the switching devices for gate driving or the ICs forgate driving as described in connection with the problems of theconventional trench IGBTs. The gate driving capabilities of theconventional devices for gate driving are not enough. Therefore, theshort circuit rate between 0 and 25% is not preferable.

When the short circuit rate is between 75 and 100%, the turn-onvoltage-change-speed rises sharply with increasing short circuit rateand the reduction of the turn-on losses is small. As described in FIG.5, the turn-on voltage-change-speed does not become as small as theshort circuit rate approaches 100%, even when a high gate resistance isused. Since it is impossible to confine the radiation noise within thespecified range, the short circuit rate between 75 and 100% is notpreferable. Therefore, the short circuit rate between 25 and 75% ispreferable.

Since emitter electrode 7 and second p-type base regions 12 areconnected electrically to each other not only in the vicinities oftrenches 21, but also on both sides of gate runner 14 in the centralpart of the active region, the gate-collector capacitance is reduced inthe entire p-type base regions 12 surrounded by the respective trenches21 in the central part of the active region. Therefore, the speed ofvoltage drop during the turn-on of the IGBT (hereinafter referred to asthe “turn-on voltage-drop-speed”) becomes high and the turn-on lossesare reduced according to the first embodiment of the invention.

By setting the short circuit rate between 25 and 75%, an IGBT having anoptimum structure that facilitates meeting the specifications on theturn-on losses and radiation noise is obtained according to the firstembodiment of the invention. Therefore, the problem of insufficient gatedriving capabilities of the conventional switching devices for gatedriving or the conventional ICs for gate driving and the problem of theradiation noise outside the specified range are prevented.

Second Embodiment

FIG. 6 is a top plan view schematically showing a trench IGBT accordingto a second embodiment of the invention. FIG. 7 is a cross sectionalview along the line segment E-E of FIG. 6 across contact holes 11aligned along a side of gate runner 14.

In the trench IGBT according to the first embodiment (shown in FIG. 1),n-type source regions 3 are formed only in first p-type base regions 9.To facilitate this, the position of the mask for forming n-type sourceregions 3 and the position of the mask for forming trenches 21 must beset very precisely relative to each other. In order to relax therequired mask positioning precision and improve the mass-productionefficiencies, it is effective to widen n-type source regions 3. In FIGS.6 and 7, n-type source regions 3 are widened and formed on both sides oftrenches 21. This configuration facilitates distributing n-type sourceregions 3 uniformly over the active region and reducing the complicationof not forming n-type source regions 3 in some p-type base regions.

Although n-type source regions 3 are formed in third p-type base region10, n-type source regions 3 in third p-type base region 10 do notfunction as sources, since n-type source regions 3 in third p-type baseregion 10 are insulated from emitter electrode 7 by interlayer insulatorfilm 6. Although n-type source regions 3 are formed also in secondp-type base region 12, n-type source regions 3 in second p-type baseregion 12 do not function as sources, since second p-type base region 12is in contact with emitter electrode 7 via contact holes 11, but n-typesource regions 3 in second p-type base region 12 are not in contact withemitter electrode 7. The trench IGBT according to the second embodimentof the invention exhibits the same effects as the trench IGBT accordingto the first embodiment.

Third Embodiment

FIG. 8 is a cross sectional view for describing the principle of atrench IGBT according to a third embodiment of the invention. Referringnow to FIG. 8, p-type base layer 20 is divided into relatively narrowp-type base regions 9 and relatively wide p-type base regions 16 widerthan relatively narrow p-type base regions 9. Relatively wide p-typebase region 16 is connected electrically to emitter electrode 7 via gateresistance 31 of 50 mΩ or higher. Emitter electrode 7 is in contactcommonly with n-type source regions 3 (in relatively narrow p-type baseregions 9) and relatively narrow p-type base region 9 in the same manneras in the trench IGBT according to the first embodiment.

FIG. 9 is a graph describing the simulated relation between the peakcurrent reduction rate and the value of gate resistance 31 in the trenchIGBT having the structure shown in FIG. 8. When the peak current valuein the structure which insulates relatively wide p-type base regions 16from emitter electrode 7 (which is identical to the conventionalstructure shown in FIG. 19) is set at 100 and when it is necessary toadjust the peak current value at 90% of the peak current value or lowerin the conventional structure, it is effective, as FIG. 9 indicates, toset the value of gate resistance 31 at 2Ω or lower. For adjusting thepeak current value at 92% of the peak current value or lower in theconventional structure, it is effective to set the value of gateresistance 31 at 3Ω or lower. For adjusting the peak current value at96% of the peak current value or lower in the conventional structure, itis effective to set the value of gate resistance 31 at 100Ω or lower.

FIG. 10 is a graph describing the simulated relation between theon-resistance and the value of gate resistance 31 in the trench IGBThaving the structure shown in FIG. 8. Referring now to FIG. 10, theon-voltage is 2.5 V or lower when the value of gate resistance 31 is 50mΩ or higher. As the value of gate resistance 31 exceeds 50 mΩ to thelower side, the on-voltage exceeds 2.5 V to the higher side, causing asteady state loss increase. Therefore, the value of gate resistance 31of 50 mΩ or lower is not practical. Therefore, the value of gateresistance 31 is preferably 50 mΩ or higher and 100Ω or lower, morepreferably 50 mΩ or higher and 3Ω or lower, and most preferably 50 mΩ orhigher and 2Ω or lower.

FIG. 11 is a wave chart describing the simulations results for comparingthe turn-on characteristics of the trench IGBTs, in which the value ofgate resistance 31 is set at 400 mΩ. As designated by the referencenumerals 41 and 42 in FIG. 11, the peak current value during the turn-on(hereinafter referred to as the “turn-on peak-current-value”) of thetrench IGBT having the structure shown in FIG. 8 is 81 A for the gateresistance set at 12Ω and 51 A for the gate resistance set at 48Ω.Therefore, the turn-on peak-current-value of the trench IGBT is loweredgreatly by changing the gate resistance value from 12Ω to 48Ω. In thiscase, the peak current reduction rate is about 37%.

As designated by the reference numerals 43 and 44 in FIG. 11, theturn-on peak-current-value of the trench IGBT having the conventionalstructure shown in FIG. 19 is 100 A for the gate resistance 31 set at12Ω and 95 A for the gate resistance 31 set at 48Ω. In this case, thepeak current reduction rate is about 5%. Since the turn-onpeak-current-value is lowered greatly according to the third embodimentas described above, the switching losses are reduced according to thethird embodiment.

The turn-on peak-current-value is almost proportional to the turn-oncurrent-change-speed (di/dt). In other words, as the peak current valueis higher, the turn-on current-change-speed is higher. As the peakcurrent value is lower, the turn-on current-change-speed is slower.Since the peak current changing rate with respect to the gate resistance31 change is larger according to the third embodiment than thataccording to the prior art, a low current-change-speed is realized bythe gate resistance 31 lower than that employed in the trench IGBTsaccording to the prior art. Therefore, radiation noise in the switchingof the trench IGBT is reduced.

Now gate resistance 31 is described in detail below. FIG. 12 is aperspective view schematically showing the structure of the trench IGBTthat employs the sheet resistance of an impurity layer in relativelywide p-type base region 16 for gate resistance 31 for connectingrelatively wide p-type base region 16 to emitter electrode 7. In thetrench IGBT including stripe-shaped trenches 21, contact sections 7 a,in which relatively narrow p-type base regions 9 and emitter electrode 7contact with each other, are formed continuously along respective p-typebase regions 9. Relatively wide p-type base region 16 is connectedelectrically to emitter electrode 7 via contact holes 7 b formed throughinterlayer insulator film 6. Contact holes 7 b are filled with the metalsputtered for forming emitter electrode 7.

Although not limited specifically, contact hole 7 b is about several μm²in area, e.g. 5 μm². Contact holes 7 b are aligned along the stripe oftrench 21 with a spacing that is between 200 μm and 2 mm. Simply bychanging a part of the mask pattern for forming the etching mask usedfor etching interlayer insulator film 6, contact holes 7 b can be formedsimultaneously with sputtering interlayer insulator film 6 for formingcontact sections 7 a. In other words, the trench IGBT according to thethird embodiment is manufactured through a manufacturing process that isthe same as the manufacturing process by which a conventional trenchIGBT is manufactured.

FIG. 13 is a graph describing the experimental results that relates thespacing between contact holes 7 b to the peak current reduction rate inthe trench IGBT having the structure shown in FIG. 12. The vertical axisof FIG. 13 represents the difference between the turn-onpeak-current-values of the trench IGBT shown in FIG. 12 for the gateresistance values of 12Ω and 48Ω. As FIG. 13 indicates, the peak-currentreduction-rate is 10% or larger for a spacing between contact holes 7 bof 2 mm or narrower. Thus, the characteristics of the trench IGBT areimproved remarkably as compared to the characteristics of theconventional trench IGBT (the peak current reduction rate of around 5%).

FIG. 14 is a graph describing the experimental results relating thespacing between contact holes 7 b and the on-voltage in the trench IGBThaving the structure shown in FIG. 12. As FIG. 14 indicates, the spacingbetween contact holes 7 b of narrower than 200 μm is not practical,since the on-voltage exceeds 2.5 V toward the higher side at the contacthole spacing of narrower than 200 μm. Therefore, it is effective to setthe spacing between the contact holes 7 b between 200 μm and 2 mm. Forexample, the spacing between contact holes 7b is 500 μm for the trenchIGBT exhibiting a breakdown voltage of the 1200 V class. The spacingbetween contact holes 7 b is 1000 μm for the trench IGBT exhibiting abreakdown voltage of the 1700 V class.

The dimensions and the impurity concentrations in the trench IGBT havingthe structure shown in FIG. 12 are described below. The breakdownvoltage class is from 600 to 3300 V. As described in FIG. 12, the unitcell width is from 20 to 30 μm. Trench 21 is 1 μm in width. The pitchbetween two trenches 21 and 21 sandwiching relatively narrow p-type baseregion 9 is from 3 to 4 μm. The spacing between two trenches 21 and 21sandwiches relatively wide p-type base region 16, that is the width ofrelatively wide p-type base region 16 is from 15 to 26 μm. The impurityconcentration in relatively wide p-type base region 16 is from 10¹⁶ cm⁻³to 10¹⁸ cm⁻³ and preferably 10¹⁷ cm⁻³.

Fourth Embodiment

FIG. 15 is a top plan view of a trench IGBT according to the fourthembodiment of the invention. FIG. 16 is a cross sectional view along theline segment F-F of FIG. 15 across relatively wide p-type base region 16and a contact section (hereinafter referred to as a “first contactsection”) 32 a, in which relatively wide p-type base region 16 and adoped polysilicon layer 32 contact each other. FIG. 17 is a crosssectional view along the line segment G-G of FIG. 15 across firstcontact section 32 a and a contact section (hereinafter referred to as a“second contact section”) 7 c, in which emitter electrode 7 and dopedpolysilicon layer 32 contact each other. According to the fourthembodiment, gate resistance 31 connecting relatively wide p-type baseregion 16 with emitter electrode 7 is made of a doped polysilicon layer32.

In FIG. 15, p-type base regions 9 and 16, gate insulator films 4, gateelectrodes 5, doped polysilicon layer 32, first contact sections 32 aand second contact section 7 c projected to the surface of dopedpolysilicon layer 32 are shown. Interlayer insulator films 6 and emitterelectrode 7 are not shown in FIG. 15. The constituent elements not shownin FIG. 15 are shown in FIGS. 16 and 17.

Doped polysilicon layer 32 is disposed above relatively wide p-type baseregion 16 and shaped with a stripe extending along trenches 21. Dopedpolysilicon layer 32 is isolated from relatively wide p-type base region16 and emitter electrode 7 by interlayer insulator film 6 but connectedelectrically to relatively wide p-type base region 16 via first contactsections 32 a. Doped polysilicon layer 32 is connected to emitterelectrode 7 via second contact section 7 c. The value of gate resistance31 shown in FIG. 8 is controlled by the distance between first andsecond contact sections 32 a and 7 c, the width of doped polysiliconlayer 32, and the impurity concentration in doped polysilicon layer 32.The trench IGBT according to the fourth embodiment exhibits the sameeffects as the trench IGBT according to the third embodiment exhibits.Gate resistance 31 can be built in the trench IGBTs without employingany multilayered metal wiring structure.

Thus, a trench IGBT has been described according to the presentinvention. In the IGBTs according to the embodiments of the invention,changes and modifications are obvious to those skilled in the artwithout departing from the true spirit of the invention. For example,multiple gate runners 14 may be disposed in the active region in theIGBT according to the first embodiment or the second embodiment. In thiscase, contact holes 11 may be formed on both sides of each gate runner14 in second p-type base region 12 and second p-type base region 12 andemitter electrode 7 may be connected electrically via contact holes 11.

The trench IGBTs according to the invention are useful in the field ofpower devices used for electric power converters.

1. An insulated gate semiconductor device comprising: a firstsemiconductor layer of a first conductivity type; a second semiconductorlayer of a second conductivity type on the first semiconductor layer; athird semiconductor layer of the first conductivity type on the secondsemiconductor layer; trenches formed through the third semiconductorlayer down to the second semiconductor layer to divide the thirdsemiconductor layer into multiple semiconductor regions; fourthsemiconductor layers of the second conductivity type selectively formedat least in the surface portions of some of the semiconductor regions; acontrol electrode formed in each of the trenches with an insulator filminterposed between the control electrodes in the trenches; a runner onthe third semiconductor layer in the active region, in which currentflows in the semiconductor device, with an insulator film interposedbetween the runner and the third semiconductor layer, and the runnerbeing connected electrically to the control electrodes; a first mainelectrode on the third and fourth semiconductor layers with aninterlayer insulator film interposed between the first main electrodeand the third and fourth semiconductor layers, the first main electrodebeing in contact with the third semiconductor layer and the fourthsemiconductor layers in the semiconductor regions that have the fourthsemiconductor layers formed therein, through the interlayer insulatorfilm, and the first main electrode being in contact with the thirdsemiconductor layer in some of the semiconductor regions that do notinclude any fourth semiconductor layer, via contact holes formed in thevicinities of the terminal ends of the trenches and in the vicinities ofthe runner through the interlayer insulator film; and a second mainelectrode connected electrically to the first semiconductor layer. 2.The insulated gate semiconductor device according to claim 1,additionally comprising semiconductor regions that do not include anyfourth semiconductor layer and that are insulated from the first mainelectrode by the interlayer insulator film.
 3. The insulated gatesemiconductor device according to claim 2, comprising a number N1 ofsemiconductor regions which are in contact with the first main electrodebut which do not include any fourth semiconductor layer and a number N2of semiconductor regions which do not include any fourth semiconductorlayer and which are insulated from the first main electrode.
 4. Theinsulated gate semiconductor device according to claim 3, wherein N1 andN2 are related according to the expression0.25≦(N1/(N1+N2)≦0.75.
 5. An insulated gate semiconductor devicecomprising: a first semiconductor layer of a first conductivity type; asecond semiconductor layer of a second conductivity type on the firstsemiconductor layer; a third semiconductor layer of the firstconductivity type on the second semiconductor layer; trenches formedthrough the third semiconductor layer down to the second semiconductorlayer to divide the third semiconductor layer into at least firstsemiconductor regions, second semiconductor regions, and thirdsemiconductor regions; fourth semiconductor layers of the secondconductivity type selectively formed at least in the surface portions ofthe first semiconductor regions; a control electrode formed in each ofthe trenches with an insulator film interposed between the controlelectrodes in the trenches; a runner on the third semiconductor layer inthe active region, in which current flows in the semiconductor device,with an insulator film interposed between the runner and the thirdsemiconductor layer, and the runner being connected electrically to thecontrol electrodes; a first main electrode on the third and fourthsemiconductor layers with an interlayer insulator film interposedbetween the first main electrode and the third and fourth semiconductorlayers, the first main electrode being in contact through the interlayerinsulator film with the third semiconductor layer and the fourthsemiconductor layers in the first semiconductor regions, the first mainelectrode being in contact with the third semiconductor layer only viacontact holes formed through the interlayer insulator film in the secondsemiconductor regions, the contact holes being formed in the vicinitiesof the terminal ends of the trenches and in the vicinities of therunner; the first main electrode being insulated from the thirdsemiconductor layer and the fourth semiconductor layers by theinterlayer insulator film in the third semiconductor regions; and asecond main electrode connected electrically to the first semiconductorlayer.
 6. The insulated gate semiconductor device according to claim 5,comprising a number N1 of second semiconductor regions and a number N2of third semiconductor regions wherein N1 and N2 are related by theexpression0.25≦(N1/(N1+N2)≦0.75.